1. Cross-Reference to Related Application
This patent application is related to U.S. patent application Ser. No. 06/130,775 filed Mar. 17, 1980 and now U.S. Pat. No. 4,377,817.
2. Field of the Invention
The present invention relates generally to a solid state image sensor, and more particularly to a semiconductor image sensor which is capable of non-destructive readout of optical information, wide in dynamic range, high in sensitivity, low in noise, excellent in spatial and time-domain resolution and very high in packing density. Further, the invention pertains to a method for operating such a semiconductor image sensor.
3. Description of the Prior Art
Solid-state image sensors are roughly divided into the CCD and the MOS type. Despite the similarities in fabrication technologies, the performance of the MOS and CCD image sensors are different because they have different methods of imaging the light and different techniques of reading out signal charges. In the CCD image sensor, signal charges are stored in a potential well induced by an electric field in a semiconductor region beneath an MOS capacitor electrode, and their readout is accomplished by multiple transfers through such field-induced potential wells to an output circuitry. But in the MOS image sensor, the signal charge is collected by a photodiode formed by diffusion or ion implantation, and the readout is carried out by a single transfer from the photodiode via an adjoining MOSFET to video-out circuitry. These differences in structure, manufacture and readout result in wide differences in performance at high and low light levels and in image clarity. At low light levels, the minimum light that can be resolved by the image sensor largely depends on the light collecting ability of the image sensor, that is, the efficiency with which the image sensor can collect the light incident thereon, as well as noise introduced by the sensor cell and its associated circuitry.
The MOS image sensor converts light to signals more efficiently than the CCD. This results from the differences in the amount of light reflected from the imaging surface of each device and in the aperture efficiency. The CCD image sensor has an array of two- or three-layer electrodes on the imaging surface, which absorb much light, so that the CCD image sensor collects less light than does the MOS image sensor.
Two common techniques are used to illuminate the semiconductor substrate in monolithic image sensors. These are front and back illumination.
Either technique could be used with either CCD or MOS image sensors but front illumination is not suitable for use with the CCDs because most CCD structures have opaque electrodes on the front which reduce the light collecting area.
Unfortunately, back lighting introduces a fabrication problem and performance limitations. In the case of the back illumination, the substrate must be made very thin so that light-generated carriers (which are usually generated within about 5 .mu.m of the semiconductor surface for visible light in the case of a silicon substrate) may be efficiently collected and stored in a depletion layer beneath capacitor electrodes on the front side. The thinnest substrate that can be fabricated has a thickness of about 25 .mu.m. This means that device elements cannot be spaced less than 25 .mu.m apart; namely, since the carriers generated by back lighting in the substrate spread by diffusion, it is necessary from the viewpoint of the spatial resolution that the thicker the substrate becomes, the more MOS capacitor electrodes on the front be spaced apart. This is a restriction that limits the potential resolution of back-illuminated CCDs. This limitation on the element spacing is especially damaging for large-capacity image sensors having a large number of picture elements because it means that a large silicon substrate is needed.
Clearly, front illumination is desirable for simple structures to give good resolution. MOS image sensors, fortunately, have a silicon oxide film layer covering the semiconductor substrate, and this transparent oxide film acts as an optical coating that matches the optical impedance of the silicon to the impedance of air.
Some CCD image sensors also have been built with polysilicon electrodes that can be illuminated from the front, but these polysilicon structures provide poor impedance matches with the oxide film beneath, which causes reflection at the polysilicon-oxide interface. These mismatches create interference patterns in the surface reflection, resulting in a decrease in the photocurrent output.
Whether the image sensor array is illuminated from the front or the back, noise introduced into the video signal by the image sensors and associated circuitry is the greatest factor that limits operation at low light levels. The noise, which masks small photosignals at low light levels, comes mainly from mismatches in parasitic capacitances and thermally generated carriers. Moreover, CCDs suffer noise from transfer losses.
In MOS image sensors, a problem arises from capacitive coupling noise that results from mismatches between parasitic gate-source and gate-drain MOS capacitance of transistors in the scanning circuit and photodiodes and video output port (with which these capacitances are in series). These MOS transistors are analog switches that address the individual photoelements in the array.
When these MOS transistors are turned on or off, there is a corresponding voltage spike on the analog photosignal line being switched. Although these spikes may be reduced by low-pass filtering, because they occur at twice the maximum video frequency, they cannot be eliminated completely.
The perturbation in the magnitude of these spikes throughout the MOS photoarray gives rise to fixed-pattern noise in the video passband. This type of noise can be eliminated by low pass filters. Fortunately, the variation in the noise is small compared to the absolute magnitude of the spikes.
Spike noise, as observed at the sensor output, is referenced to an equivalent noise voltage across the capacitance of the photosensing element, for example, in a representative 512-element line sensor. Values of noise range from 1.times.10.sup.-3 to 0.5.times.10.sup.-2 volts, well within practical operating levels. The saturated output signal referred to the photodiode is typically 5 volts or so, resulting in dynamic ranges of 100 to 1 and more.
While CCDs are not affected by the fixed-pattern noise from the spikes in switching transistors, they have fixed-pattern noise resulting from capacitance between clock lines and the output lines. Luckily, these noise pulses are all the same height and can be filtered out by low-pass filters, but the filters consume power and occupy space.
The best method of reducing this capacitive coupling noise is to fabricate video preamplifiers on the same image sensor chips. The noise is thereby reduced because the magnitude of the parasitic coupling capacitance may be made smaller for amplifiers on the same chips than for off-the-chip amplifiers.
Fixed-pattern noise in both MOS image sensors and CCDs can also come from thermal effects (thermally generated carriers). The CCD image sensors, however, are more susceptible to thermal effects than are the MOS image sensors because the surface of the CCDs is not in equilibrium, which causes thermal imbalance.
This form of noise is most troublesome at illumination levels below 10 .mu.W/cm.sup.2 and for light-integration periods longer than 100 msec for typical image sensors because the noise comprises a significant portion of the dark current at these levels and represents the ultimate operating limitation.
But with the CCD image sensors, transfer-loss noise is more damaging than the fixed-pattern noise. This reduces the exposure range of the CCD image sensors and consequently decreases the contrast that they can detect.
Now, the conventional image sensors will be evaluated from the standpoints of (1) dynamic range, (2) sensitivity, (3) noise and (4) image clarity which are important performance criteria. In ordinary image sensors, the lower limit of the dynamic range depends on the spike noise resulting from the aforesaid capacitive coupling and the thermal noise (dark-current noise), and the upper limit is, in the MOS image sensors, the bias voltage of the photodiodes and, in the CCDs, the depth of the potential well (both of which are about 5 V). In terms of (2), sensitivity, letting the capacitance for storing carriers and the amount of charges being stored be represented by C.sub.S and Q, respectively, the stored voltage is given by Q/C.sub.S. This photodiode voltage is capacitively divided by the output line capacitance C.sub.B and output voltage is represented as the following voltage: ##EQU1## where C.sub.B is the capacitance of the output line. That is, the stored voltage Q/C.sub.S is divided by the sum of the capacitance C.sub.B of the output line and the storage capacitance C.sub.S and the output voltage is reduced to Q/(C.sub.B +C.sub.S). Further, it is a matter of course that the influence of noise must be taken into account when discussing the sensitivity.
Moreover, the sensitivity depends on the amount of light that is sensed by sensor cells, that is, their light collecting ability. The problem of sensitivity should be considered from the view points of unevenness of the illuminated surface and also the impedance matching. The most critical is the spike noise that results from the capacitance coupling and ranges from 10.sup.-3 to 0.5.times.10.sup.-2 V and defines the lower limits of the dynamic range. The dark-current noise poses a problem as the light integration period becomes longer (for example, in excess of 100 msec) at low light levels below 10 .mu.W/cm.sup.2 and it limits the operational range of the image sensor.
In terms of (4), spatial resolution, the smaller the cell area, the better. In practice, however, for example, the back illumination type CCD image sensors must be designed so that the electrodes are spaced more than the thickness of the substrate apart. Further, since the voltage Q/C.sub.S stored in the storage capacitance C.sub.S of the image cell is read out in the form of ##EQU2## reduced by the capacitive division with the output line C.sub.B, operational amplifiers with high sensitivity and low noise are needed and a minimum area of the image cells is limited, which depends on the sensitivity of the operational amplifiers and the noise level. The light integration period which is determined by a reciprocal of the frame frequency gives temporal resolution (time-wise image clarity).
The CCD image sensors have such a serious drawback that since information is transferred through the cell structure, if even one of the image cells is defective the signal intensity of all image cells preceding the defective one are subject to changes. Accordingly, all the cells must be fabricated defect-free, but such cells are difficult to manufacture with good yield.
In contrast thereto, the MOS image sensor which can be read out by the random access system is free from the above-said defect resulting from the charge transfer. Since the MOS image sensor has the arrangement that charges stored in photodiodes are read out, however, it is diffult to raise the sensitivity, and an amplifier with very low noise and high sensitivity is required.
The present inventors have previously proposed in the aforementioned U.S. patent application Ser. No. 06/130,775 (U.S. Pat. No. 4,377,817) a novel image sensor which has incorporated therein a hook structure having an amplifying function with a view toward removing the aforesaid defects of the prior art CCD and MOS image sensors, in particular, increasing the light detecting sensitivity. This novel image sensor in an improvement over the CCD and MOS image sensors, in its capability for highly sensitive random access of optical information.
The present inventors have further improved the operational characteristics of this novel image sensor and proposed in the aforesaid U.S. patent application an image sensor designed to have an optimum structure for the non-destructive readout operation. This image sensor has each cell comprised of a hook structure, a readout transistor and a refresh transistor but possesses the defects that the cell structure is complex due to the provision of the refresh transistor and hence is difficult to produce and that high packing density is difficult to achieve especially when fabricating a large capacity image sensor.